Modalities in homotopy type theory

Univalent homotopy type theory (HoTT) may be seen as a language for the category of $\infty$-groupoids. It is being developed as a new foundation for mathematics and as an internal language for (elementary) higher toposes. We develop the theory of factorization systems, reflective subuniverses, and modalities in homotopy type theory, including their construction using a "localization" higher inductive type. This produces in particular the ($n$-connected, $n$-truncated) factorization system as well as internal presentations of subtoposes, through lex modalities. We also develop the semantics of these constructions

Accessible higher modalities

Treballs finals del Màster en Matemàtica Avançada, Facultat de Matemàtiques, Universitat de Barcelona: Curs: 2022-2023. Director: Carles Casacuberta[en] The main goal of this master’s thesis is to study localizations in the setting of homotopy type theory. In particular the special case of modalities, an object resembling the namesake notion in classical logic that allows to qualify statements to express things as possibility or necessity. In the first chapter we introduce the syntax of homotopy type theory and some motivating examples for the study of modalities. Next, we study localizations and modalities as described in [1; 2]. We focus on the fact that accessible modalities correspond to nullifications and work on the existence of non-accessible modalities. Finally we explore the use of Agda (a programming language for automatic proof verification) towards implementing results about factorization systems and modalities

Goodwillie's Calculus of Functors and Higher Topos Theory

We develop an approach to Goodwillie's calculus of functors using the techniques of higher topos theory. Central to our method is the introduction of the notion of fiberwise orthogonality, a strengthening of ordinary orthogonality which allows us to give a number of useful characterizations of the class of $n$-excisive maps. We use these results to show that the pushout product of a $P_n$-equivalence with a $P_m$-equivalence is a $P_{m+n+1}$-equivalence. Then, building on our previous work, we prove a Blakers-Massey type theorem for the Goodwillie tower. We show how to use the resulting techniques to rederive some foundational theorems in the subject, such as delooping of homogeneous functors.Comment: 40 pages, (a slightly modified version of) this paper is accepted for publication by the Journal of Topolog